It is sought more particularly here below in this document to describe problems existing in the field of seismic data acquisition for oil prospecting industry. The present disclosure of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
The operations of acquiring seismic data on site conventionally use networks of seismic sensors, like accelerometers, geophones or hydrophones. In a context of seismic data acquisition in a marine environment, these sensors are distributed along cables in order to form linear acoustic antennas normally referred to as “streamers” or “seismic streamers”. As shown in FIG. 1, several streamers S1-S4 in parallel form a network of seismic streamers which is towed by a seismic vessel V.
The seismic method is based on analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate omni-directional seismic wave trains. The pressure wave generated by the seismic sources passes through the column of water and insonifies the different layers of the sea bed. Part of the seismic waves (i.e. acoustic signals) reflected are then detected by the hydrophones distributed over the length of the seismic streamers. These seismic acoustic signals are processed and retransmitted by telemetry from the seismic streamers to the operator station situated on the seismic vessel, where the processing of the raw data is carried out (in an alternative solution, the seismic acoustic signals are stored for a later processing).
During seismic surveys, it is important to precisely locate the streamers in particular for:                monitoring the position of the hydrophones (distributed along the seismic streamers) in order to obtain a satisfactory precision of the image of the sea bed in the exploration zone;        detecting the movements of the streamers with respect to one another (the streamers are often subjected to various external natural constrains of variable magnitude, such as the wind, waves, currents); and        monitoring the navigation of streamers, in particular in a situation of bypassing an obstacle (such as an oil barge).        
This function is ensured by the acoustic positioning system, which comprise acoustic nodes, arranged along the streamers (they are regularly plugged externally or inline to the streamers), and a master controller system.
As also shown in FIG. 1, paravanes (or “doors”) P1-P2 are hydrodynamic foils which are disposed laterally outwardly on each side of the plurality of streamers S1-S4, and allow to maintain lateral separation between adjacent streamers.
We detail now which it is important to know the paravane's water speed and the outer streamers' water speed.
Traditionally, seismic vessels sail in a straight line over a target, then turning back to shoot another line parallel to the first line. A well-known problem is to monitor in turn the drag force of the paravane which is external to the turn. The drag force has to be compatible with the rigging specifications, i.e. with the main line of the paravane handling system. Another problem is to monitor in turn the lateral force produced by the paravane which is internal to the turn. If the lateral force is too low, then the lateral separation between streamers may not be sufficient. The lateral force and the drag force mainly depend on the vessel water speed, the turn radius and the lateral separation between the paravane and the vessel.
Actually, the lateral force and the drag force are monitored with two independent measures, the towing rope tension of the paravane and the outer streamers water-speed. A high threshold on the towing rope tension ensures that the drag force of the paravane is acceptable and a low threshold ensures that the lateral force generated by the paravane is sufficient to maintain a lateral separation between adjacent streamers.
Some paravanes used in operation have foils with a height of 10 m, suspended below cylindrical floats more than 9 m in length. It is recurrent that object like tree's branch or trunk are caught by the paravanes, increasing significantly the drag force. In that case the water speed of the paravane allows to check coherence of the tension measurement and can permit to determine if a high tension is caused by an object caught by the paravane. In other words, in addition to the towing rope tension measurement, the paravane's water speed has to be known in order to identify the cause of a high tension value. Indeed, in addition to the vessel water speed, to the turn radius and to the lateral separation of the paravane, an object caught by the paravane may increase significantly the drag force.
As well, it cannot be assumed that the paravane's lateral force is always proportional to the tension on the paravane's towing rope. This is the case in ideal conditions, but some events may alter this hypothesis. For example a low lateral force may be hidden by an object caught by the paravane. In that case the paravane's water-speed is not affected and still traduces the lateral force applied by the door.
The paravane's water speed is traditionally measured by a battery powered instrument, called “speedlog”, which is plugged on the outer streamers, close to the paravane.
A drawback of this specific measure instrument is that it requires a regular maintenance, in order to change the batteries, to clean the sensor and to check the calibration.
Another drawback of this specific measure instrument is that, when it is used to measure the water speed of a streamer, it only gives the water-speed in one axis, the streamer axis. So it cannot be used for predicting the streamers distortion, which is useful for managing the streamer network shape. Actually, the streamer distortion is mainly estimated with a current meter placed on the vessel's hull, also call ADCP (Acoustic Doppler Current Profiler), and a predictive algorithm which allows to estimate the current which will be seen by the streamer when it will reach the vessel position. The longer are the streamers, the worst are the current predictions at the streamer's tails because of the time which separates the ADCP measure and the time at which the streamer will be at the ADCP position of the measure.